How Seedlings See the Light

Seedlings start to turn green just minutes after they emerge from the ground. Exactly how light touches off this transformation has long been a mystery. Now, a team of scientists has uncovered a remarkably simple pathway that kick-starts this process.

The master regulator of this so-called photomorphogenesis is a protein called COP1, discovered a decade ago by Xing-Wang Deng's team at Yale University. COP1 works hard while seedlings are in the dark to keep photomorphogenesis genes turned off. It does this by making sure that no relevant transcription factors are around to make RNA from the genes. When seedlings encounter light, supplies of COP1 dwindle in the nucleus, allowing transcription factor levels to rise and switch on photomorphogenesis genes.

But how does COP1 learn that the lights are on? In a report published online this week by Science, Deng and his colleagues suggest that in the plant Arabidopsis thaliana, blue-light photoreceptor proteins known as cryptochromes cozy up to COP1. A battery of tests showed that the two proteins can directly interact with each other: For example, antibodies against COP1 also fished a common class of cryptochromes out of Arabidopsis extracts. The case for direct interaction was bolstered by similarities between gene expression profiles of plants with various deficiencies in photoreceptors and COP1.

The team proposes that blue light alters cryptochromes so that they can attach to COP1. This stops the protein from its normal business of tagging transcription factors for degradation. As a result, the gene-transcribing machinery, which is ready and waiting in the nucleus, can respond almost instantaneously to light-signal changes.

Plant geneticist Peter Quail of the University of California, Berkeley, says the process is unexpectedly simple: There could easily have been a dozen regulators between cryptochromes and COP1, he says. "It's a very dramatic shortening of the potential pathways."